With the advent of relatively small rechargeable dry batteries, and generally longer life dry batteries, battery operated portable hand held appliances which include both operator control of motor speed and substantial variation in motor load e.g., drills, routers, screwdrivers, etc., have been developed and widely used. User satisfaction demands that portable appliances be responsive to operator and provide consistent, reproducible results so long as the battery source has a usable level of charge at the time of use. Additionally, motor control circuitry for portable applications must have a reasonably long service life; must not be wasteful of power; and must be cost effective.
Early battery operated appliances rely heavily on mechanical switches and variable resistances to achieve motor control. While such devices are generally reliable, they tend to be unresponsive to changes in motor load, and they tend to dissipate excessive amounts of power in the control circuitry.
Some power tools have used tachometers to monitor speed and provide feedback control signals; such arrangements are both costly and bulky.